How to remediate – Server-Side Template Injection

1. Introduction

Server-Side Template Injection (SSTI) is a vulnerability where an application embeds user-controlled inputs into server templates without proper sanitisation, allowing attackers to inject code that is then executed by the server. This can lead to sensitive information disclosure or remote code execution on the affected system. Web applications using template engines like Twig, Jinja2, and Freemarker are commonly impacted. Successful exploitation could compromise confidentiality, integrity, and availability.

2. Technical Explanation

• Root cause: Unsafe inclusion of user-controlled data within server templates without proper validation or escaping.
• Exploit mechanism: An attacker submits a crafted payload containing template engine syntax, which is then executed by the server when rendering the page. For example, in Jinja2, an attacker might inject {{7*7}} to trigger code execution.
• Scope: Applications using any server-side template engine are potentially affected, including those built with Python (Jinja2), PHP (Twig, Smarty), Java (Freemarker), and Ruby on Rails (ERB).

3. Detection and Assessment

Confirming a vulnerability requires identifying if user input is directly included in templates and whether the template engine allows code execution. A quick check involves examining application source code or configuration files for template usage. Thorough assessment includes attempting to inject test payloads.

• Quick checks: Review application configuration files (e.g., application.conf, settings.py) for references to template engines and file paths.
• Scanning: Burp Suite’s Intruder can be used with a payload list of common SSTI payloads to test various injection points. This is an example only; results require manual verification.
• Logs and evidence: Look for error messages related to template parsing or execution errors in application logs, particularly those containing user input.

grep -r "template" /path/to/application/sourcecode

4. Solution / Remediation Steps

The most effective solution is to avoid using user inputs directly within server templates. If unavoidable, use logic-less template engines or create sandboxed environments.

4.1 Preparation

• Ensure a rollback plan is in place by keeping a copy of the original configuration files or code. A change window may be required, depending on service criticality.

4.2 Implementation

1. Step 1: Identify all instances where user input is used within server templates.
2. Step 2: Replace direct inclusion of user input with safe alternatives like pre-defined variables or whitelisted data.
3. Step 3: If using a template engine, switch to a logic-less engine if feasible.
4. Step 4: Alternatively, implement sandboxing mechanisms within the template engine configuration to restrict code execution.

4.3 Config or Code Example

Before

After

4.4 Security Practices Relevant to This Vulnerability

• Practice 2: Least privilege limits the impact of successful exploitation by restricting the permissions available to the application process.

4.5 Automation (Optional)

Static analysis tools can help identify instances where user input is used within templates, but manual review is still required.

# Example using grep to find potential SSTI vulnerabilities in Python code
grep -r "render_template" /path/to/application/sourcecode

5. Verification / Validation

• Post-fix check: Attempt to inject a simple SSTI payload (e.g., {{7*7}}) into an input field; confirm it is displayed as text, not executed.
• Re-test: Re-run the scanning process described in Section 3 and verify that no vulnerabilities are detected.
• Monitoring: Monitor application logs for any errors related to template parsing or execution; create an alert if such errors occur.

Attempt injection of {{7*7}} and verify it is rendered as text in the response.

6. Preventive Measures and Monitoring

Implement security baselines that enforce input validation, safe defaults, and regular patching to prevent SSTI vulnerabilities.

• Baselines: Update security baselines or policies to include requirements for secure coding practices, such as input validation and output encoding.
• Pipelines: Integrate Static Application Security Testing (SAST) tools into the CI/CD pipeline to identify potential SSTI vulnerabilities during development.
• Asset and patch process: Establish a regular patch management cycle to ensure that all software components are up-to-date with the latest security fixes.

7. Risks, Side Effects, and Roll Back

• Risk or side effect 1: Switching to a logic-less template engine may require significant code changes.
• Risk or side effect 2: Sandboxing mechanisms can sometimes introduce performance overhead.
• Roll back: Restore the application code and database from the backup created in Step 4.1. Restart affected web services.

8. References and Resources

• Vendor advisory or bulletin: N/A – This is a general vulnerability type, not specific to one vendor.
• NVD or CVE entry: https://cwe.mitre.org/data/definitions/1336
• Product or platform documentation relevant to the fix: Refer to the documentation for your specific template engine (e.g., Jinja2, Twig).